Zircon/zirconia mix for refractory coatings and inks

ABSTRACT

Refractory coatings comprising unstabilized zirconia, silica, and, optionally, zircon and/or mullite are disclosed herein. The unstabilized zirconia, silica, and optional zircon and/or mullite are applied as a slurry onto ceramic substrates such as silicon carbide and fired. The refractory coatings of the present invention maintained good edge definition and color when applied to ceramic substrates and subjected to temperatures over 1100° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of Ser. No. 10/154,134,now U.S. Pat. No. 6,753,089, filed May 23, 2002.

This invention relates to coatings on refractory materials, inparticular, zircon/zirconia coatings on ceramic substrates.

High temperature coatings for ceramics, such as silicon carbide (SiC),silicon nitride (Si₃N₄), are prone to failure in oxidizing conditions atelevated temperatures due to interactions with a growing silicon dioxide(SiO₂) glass layer. The most stable candidates for coatings are found tobe alumina-(Al₂O), mullite-(3Al₂O₃·2SiO₂), and zirconia-(ZrO₂) basedformulations due to their refractory nature, similar coefficients ofthermal expansion, and chemical stability in contact with siliconcarbide, silicon nitride, and silicon dioxide.

At temperatures above 1200° C., porous alumina coatings will dissolveinto the growing silicon dioxide layer. The resulting aluminosilicateglass fills the pores of the alumina coating creating an impermeablemembrane. This membrane traps gaseous by-products of the oxidationreaction, such as carbon dioxide. The filled pores coalesce causing thecoating to blister or flake off. Dense alumina coatings would tend tocrack and craze due to the higher coefficient of thermal expansion. Whenthe alumina coating is used in an ink application, the dissolution ofthe alumina coating bleeds into the glass leading to bleed-out of thewhite coating along the edges. For thin coatings, this is especiallyproblematic as they will either fully dissolve into the glass layer orotherwise become invisible.

Similarly, mullite coatings on silicon carbide/silicon nitride ceramicsare susceptible to blistering and flaking, and also become translucentwhen a substantial quantity of the glass layer is present. Stabilizedzirconia coatings are known to be chemically stable and have limitedsolubility in glass. However, zirconia, stabilized with calcium oryttria, were found to flake off the silicon carbide parts after one ortwo high temperature cycles at or about 1100° C.

The prior art discloses a number of coatings for refractory substrates.U.S. Pat. Nos. 4,804,589 and 4,921,721, both to Matsui et al., disclosea coating for metals and silicon carbide substrates consistingessentially of zirconia partially or fully stabilized with yttria,magnesium oxide, or calcium oxide. The coatings are deliberately keptthin to decrease peeling and crazing. A compatibilizing layer may beadded between the zirconia layer and the substrate. Matsui et al.further requires a surface pre-treatment for silicon carbide substratesfor surface roughening or reactivity enhancement. The practicality ofcoating refractory substrates with the current coating composition isdiminished with the need for surface pre-treatments. Also, thickercoatings are prone to peeling and cracking.

U.S. Pat. No. 4,950,558 to Sarin discloses a graded coating for siliconbased substrates, the coating comprising multiple layers: one or moreintermediate layers of aluminum nitride or aluminum oxynitride material,and an outer layer of an oxide of aluminum, zirconium, or yttrium. Thecoatings are prepared using chemical vapor deposition with a mixture ofgases in a continuous deposition process in which the reactant gases arechanged gradually to provide the graded layers. It would be desirable toprovide a coating which does not have the environmental concernsassociated with chemical vapor deposition.

U.S. Pat. Nos. 6,165,594, 6,214,250, and 6,251,212, all to Moh et al.,disclose a label for metal and ceramic substrates. The label includes aceramic body as a base layer which contains a glassy phase which wetsthe substrate and a refractory phase with light or dark particles; a toplayer also contains a glassy phase and a refractory phase withcontrasting particles. The color contrast between the top layer and theceramic body allows for an optically discernible labeling pattern.However, the individual labels must be formed and fired prior to beingattached to a substrate.

Notwithstanding the state of the art, it is desirable to provide coatingcompositions for high temperature refractory ceramic substrates whichprovide ease of use, good adhesion, and stability in an oxidizingatmosphere.

The present invention is directed to, in a first aspect, a refractorycoating composition for coating high temperature substrates, thecomposition comprising: unstabilized zirconia; and silica. Preferably,the unstabilized zirconia is present in an amount of about 50 to about90 parts per hundred of the composition. Preferably, the silica ispresent in an amount of about 10 to about 50 parts per hundred of thecomposition. The composition may further include zircon which may bepresent in an amount of up to 100 parts per hundred of the composition.The composition may also include an inorganic filler such as mullitepresent in an amount of up to about 50 parts per hundred of thecomposition. In a preferred embodiment of the composition of the firstaspect, the unstabilized zirconia and the silica are present in a weightratio of about 9:1 to about 1:1. The composition is useful for coatingsubstrates at temperatures greater than about 1100° C.

In a second aspect, the present invention is directed to a refractorycoating composition comprising: about 15 to about 75 parts per hundredunstabilized zirconia; about 5 to about 25 parts per hundred silica; andup to about 100 parts per hundred zircon. The composition may be appliedas a slurry or as a decal on a substrate. Preferably, the composition isapplied as a thin film having a thickness of about 20 to about 500microns.

In a third aspect, the present invention is directed to a ceramicsintered member comprising: a ceramic body; and a refractory coatingformed on a surface of the ceramic body, the refractory coatingcomprising: unstabilized zirconia; silica; and zircon, wherein therefractory coating maintains stability at temperatures in excess ofabout 1100° C. Preferably, the ceramic body comprises silicon carbide orsilicon nitride.

In a fourth aspect, the present invention is directed to a method ofmaking a ceramic sintered body comprising the steps of: providing aceramic substrate; providing a refractory coating compositioncomprising: unstabilized zirconia; silica; and zircon; applying therefractory coating composition on the ceramic substrate; and exposingthe coated ceramic substrate to sintering conditions, wherein therefractory coating on the ceramic substrate maintains stability attemperatures greater than about 1200° C. Preferably, the ceramicsubstrate comprises silicon carbide or silicon nitride. The refractorycoating composition may be applied to a portion of the ceramicsubstrate. The refractory coating composition and the ceramic substratemay be different colors and the composition is applied to a portion ofthe ceramic substrate as a marker. Preferably, the refractory coatingcomposition is painted, spray coated, sponged, brush coated, or screenprinted on the ceramic substrate.

The present invention relates to refractory coating compositions forcoating high temperature substrates including unstabilized zirconia,silicon dioxide or silica, and optionally, zircon. Unexpectedly, thecoating compositions of the present invention maintain good adhesion torefractory substrates at temperatures up to and over about 1100° C.preferably, over 1200° C., and more preferably over 1400° C. Thesecoating compositions are particularly useful as inks for labelingrefractory substrates. When used as an ink on refractory substrates, thecoating compositions demonstrated clean lines which did not bleed intothe substrate and maintained a good contrast with the substrate, evenafter multiple heat cycles. As used herein, zircon shall mean ZrSiO₄and/or its decomposition products SiO₂ and ZrO₂.

The refractory coating compositions of the present invention may beexpressed in terms of parts per hundred by weight where the total weightof the composition is equal to a hundred (100) parts and the sum of theconstituent parts equal to a hundred parts. A preferable composition mayhave from about zero to 100 parts per hundred zircon. Where the amountof zircon is less than 100 parts per hundred, the remainder of thecomposition may include about 50 to about 90 parts per hundredunstabilized zirconia, preferably about 15 to about 75 parts perhundred, and about 10 to about 50 parts per hundred silica, preferablyabout 5 to about 25 parts per hundred. Preferably, the unstabilizedzirconia and silica are in a weight ratio of about 9:1 to about 1:1,more preferably about 4:1 to about 2:1, and most preferably about 3:1.

It is hypothesized that at the extreme temperatures at which therefractory coatings are subjected to, the unstabilized zirconia whencombined with silica, forms zircon which has low solubility in thesilicon dioxide layer and is physically stable when in contact with thesilicon dioxide.

The unstabilized zirconia suitable for use in the coating compositionsof the present invention is preferably high purity monoclinic zirconia.Monoclinic zirconia is one of three crystal structures of zirconia thatis stable at temperatures of less then 1150° C.; at temperatures above1150° C., the zirconia has a tetragonal crystal structure capable of amartensitic transformation back to the monocinic crystal structureinvolving a 3 to 6 percent volume expansion when the tetragonal zirconiais not stabilized. The average particle size of the monoclinic zirconiais about 0.1 to about 200 microns, preferably 0.5 to about 2.0 microns,more preferably about 1.0 to about 10 microns, and most preferably about45 microns. A particularly desirable unstabilized zirconia iscommercially available under the trade number ZIRCOA A-325 fromSaint-Gobain/Norton Industrial Ceramics Corporation, Worcester, Mass.Colored zirconia may be substituted with or used in combination with themonoclinic zirconia to create colored coatings and inks.

It is within the scope of the invention that partially or fullystabilized zirconia, if combined with sufficient zircon or silica, canbe made into a refractory coating composition. It is hypothesized thatthe partially or fully stabilized zirconia react with the silica to formthe more stable zircon. Alternatively, if there is sufficient zirconpresent, the stabilized zirconia will be less prone to dissolve into thegrowing silicon dioxide layer.

Silica suitable for use in the coating compositions of the presentinvention is an amorphous silica. Preferably, the amorphous silica hasan average particle size of about 0.1 to about 1.0 micron, preferablyabout 0.5 microns, of spherical primary particles and agglomerates. Thespecific surface area is preferably about 15 to about 30 m²/g. Aparticularly desirable commercially available silica is MICROSILICA®from Elkem Materials, Norway. Numerous grades are available forrefractory coatings such as Grade 983, Grade 971, Grade 965, and Grade940, with Grade 971 being especially preferred. Preferably, a densifiedmicrosilica is used that contains loosely bonded secondary agglomerateswhich increase the bulk density to about 500 to about 650 kg/m³, andimprove handling characteristics of the material.

Zircon useful in the coating compositions of the present invention hasan average particle size of about 10 to about 20 microns, preferablyabout 12.8 microns. The surface area is about 0.8 to about 2.0 m²/g,preferably about 1.2 m²/g. A particularly desirable zircon iscommercially available from Saint-Gobain/Norton Industrial CeramicsCorporation of Worcester, Mass. under the trade name ZIRCON G. Coloredzircon may also be substituted for or used in combination with thezircon to create colored coatings and inks.

The composition may also include inorganic fillers which do not reactwith the silica and are tolerant of the extreme temperatures, i.e., donot burn off. Such inorganic fillers include, for example, mullite. Asused herein, mullite shall mean 3Al₂O₃.2SiO₂. The mullite may preferablybe present in an amount of up to about 50 parts per hundred parts of thetotal composition. It may also be used in place of the zircon. Thus, oneembodiment of the present invention may include up to about 50 parts perhundred mullite, and the remainder of the composition may include about25 to about 90 parts per hundred unstabilized zirconia, preferably about45 to about 50 parts per hundred, and about 5 to about 50 parts perhundred silica, preferably about 10 to about 25 parts per hundred.Again, the unstabilized zirconia and silica are preferably in a weightratio of about 9:1 to about 1:1, more preferably about 4:1 to about 2:1,and most preferably about 3:1.

A particularly suitable mullite is commercially available fromWashington Mills Electro Minerals Company, Niagra Fall, N.Y., under thetrade number DURAMUL™. Preferably, there is an excess of aluminum oxidein the mullite to ensure that all of the silicon dioxide is containedwithin the mullite phase and is not present in the glass phase. Theaverage particle size of the mullite is about 0.2 to about 250 microns,preferably about 1.0 to about 100 microns, and more preferably about 2.0to about 45 microns. A most preferred mullite is DURAMUL™ 325/F.

The unstabilized zirconia, silica, and optional zircon and mullite, arepreferably applied onto the substrate surface as a slurry in anappropriate medium. The slurry medium may be organic or inorganic. Wateris a preferred slurry medium for suspending the constituents of therefractory coating of the present invention. The unstabilized zirconia,silica, and optional zircon and mullite may be kept in suspension usingone or more stabilizers such as thickeners, thixotrophic agents, orother rheology modifiers to provide an appropriately viscous ink orcoating. A preferred stabilizer for an aqueous slurry is KELCOLOID LVFfrom Monsanto Company, St. Louis, Mo. Preferably, the stabilizer ispresent in an amount of about 0.1 to about 2.0 weight percent,preferably about 1.2 weight percent. Thus, an exemplary slurry mediummay consist of 98.8 weight percent deionized water and 1.2 weightpercent KELCOLOID LVF. Another method of keeping the coating compositionconstituents suspended in an aqueous slurry includes pH adjustment withan acid or base to achieve a stable or semi-stable electrostaticdispersion. Stabilizers may also be added to the electrostaticdispersion to ensure that the constituents of the coating remainsuspended in the liquid.

The unstabilized zirconia, silica, and optional zircon and mullite areadded to the slurry medium and mixed for a sufficient time to achieve auniform suspension. Typically, the mixing time is at least about 10 toabout 15 minutes although longer mixing times of an hour are notatypical as longer mixing times do not harm the ink or coatingcomposition. The slurry contains about 40 to about 60 weight percentsolids.

The slurry is preferably of a consistency where it may be easily appliedto the substrate depending upon the method of application. Methods ofapplying the slurry to the substrate include painting, smearing, brushcoating, spray coating, sponging, screen printing, and the like.Depending upon the application, the coating composition may be appliedin a thickness of about 20 to about 500 microns when painted on. Forexample, when applying a decal, the coating composition maybe applied ina thickness of about 50 microns using screen printing techniques with a325 mesh. One of skill in the art will understand when and how to usethe different techniques for applying the coating composition and inwhat thickness without undue experimentation. Multiple layers ofcoatings may also be applied when appropriate. Upon application of thecoating composition, the substrate is heated to drive off the slurrymedium prior to firing the ink or coating onto the substrate.

EXAMPLES

The following examples describes the preparation and use of therefractory coating compositions of the present invention. These exampleare presented for the purpose of further illustrating and explaining theinvention, and are not to be taken as limiting the scope of theinvention.

A 100 gram sample of the total constituents in parts per hundred byweight of each coating composition in their respective amounts weresuspended in a 1.0 weight percent aqueous solution of KELCOLOID LVF. Theconstituents of the coating compositions were added to the KELCOLOID LVFsolution in a water tight mixing jug and rolled with inert mixing mediato ensure adequate mixing and suspension for about an hour. Each coatingcomposition had about 40 weight percent solids. Each heat cycle wasabout 72 hours with an average rate of heating at about 75° C./hour to amaximum temperature of about 1420° C. followed by controlled cooling toroom temperature.

Example 1 (Comparative)

A comparative coating composition was formulated with 50.0 parts byweight A-15 alumina obtained from Alcoa, Inc., Pittsburgh, Pa., and 50parts by weight yttria-stabilized zirconia slurried with the KELCOLOIDLVF solution. The composition was brushed onto the surface of siliconnitride bonded silicon carbide tile, using an artist's brush in athickness of about 100 microns and the sample was dried to remove waterfrom the slurry. After 5 heat cycles, about 25% of the compositionflaked away after the first heat cycle. The remaining composition formedsmall brown blisters on the tile.

Example 2 (Comparative)

A comparative coating composition was formulated with 100 parts byweight yttria-stabilized zirconia having a mesh size of 20F. Thestabilized zirconia coating composition was slurried in the KELCOLOIDLVF solution. All but about 10% of the coating flaked off. The remainingportion had good edge definition although some grains were too coarseand did not provide a smooth surface.

Example 3

A coating composition of the present invention was formulated with 100parts by weight ZIRCON G from slurried in the KELCOLOID LVF solution.The composition was brushed onto a silicon nitride bonded siliconcarbide tile in a thickness of about 100 microns using an artist'sbrush. After three heat cycles, the composition maintained good edgedefinition and color contrast with the tile.

Example 4

A coating composition of the present invention was formulated with 56.25parts by weight unstabilized zirconia, 18.75 parts by weight densifiedMICROSILICA Grade 971 from Elkem Materials, and 25.0 parts by weightZIRCON G, and slurried in the KELCOLOID LVF solution. The compositionwas brushed onto a silicon nitride bonded silicon carbide tile. Afterthree heat cycles, the composition maintained good edge definition andcolor contrast with the tile.

Example 5

A coating composition of the present invention was formulated with 37.5parts by weight unstabilized zirconia, 12.5 parts by weight densifiedMICROSILICA Grade 971 from Elkem Materials, and 50.0 parts by weightZIRCON G, and slurried in the KELCOLOID solution. The composition wasbrushed onto a silicon nitride bonded silicon carbide tile. After threeheat cycles, the composition maintained good edge definition and colorcontrast with the tile.

Example 6

A coating composition of the present invention was formulated with 18.75parts by weight unstabilized zirconia, 6.25 parts by weight densifiedMICROSILICA Grade 971 from Elkem Materials, and 75.0 parts by weightZIRCON G and slurried in the KELCOLOID LVF solution. The composition wasbrushed onto a silicon nitride bonded silicon carbide tile. After threeheat cycles, the composition maintained good edge definition and colorcontrast with the tile.

Example 7

A coating composition of the present invention was formulated with 37.5parts by weight unstabilized zirconia, 12.5 parts by weight densifiedMICROSILICA® Grade 971 from Elkem Materials, and 50.0 parts by weightDURAMUL™ mullite from Washington Mills Electro Minerals Company. Thecomposition was slurried in the KELCOLOID LVF solution and brushed ontoa silicon nitride bonded silicon carbide tile. After three heat cycles,the composition maintained good edge definition and color contrast withthe tile.

The refractory coating compositions of the present invention provideenhanced adhesion with good edge definition when applied to a refractorysubstrate. Advantageously, the coating compositions do not flake offwhen subjected to multiple heat cycles at extreme temperatures over1100° C. nor do they dissolve into the growing silicon dioxide layerwhen used in an oxidizing atmosphere.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

1. A ceramic member, comprising: a ceramic substrate comprising siliconcarbide or silicon nitride; and a decal provided on a portion of thesubstrate as a marker for labeling the substrate, the decal having adifferent color than that of the substrate and having good edgedefinition after a heat cycle during which the ceramic member is exposedto a temperature of at least 1100° C.
 2. The ceramic member of claim 1,wherein the decal maintains good edge definition after repeated heatcycles.
 3. The ceramic member of claim 1, wherein the decal has athickness not less than about 20 microns.
 4. The ceramic member of claim2, wherein the decal has a thickness not less than about 50 microns. 5.The ceramic member of claim 1, wherein the decal has good contrast withthe ceramic substrate.
 6. The ceramic member of claim 1, wherein thedecal remains stable, having good adhesion to the ceramic substrateafter the heat cycle.
 7. The ceramic member of claim 1, wherein thedecal has clean lines that do not bleed into the ceramic substrate andmaintains good contrast with the ceramic substrate.
 8. The ceramicmember of claim 1, wherein the decal is comprised of a fired coloredink.
 9. The ceramic member of claim 1, wherein the substrate comprisessilicon carbide.
 10. A ceramic member comprising: a ceramic substrate;and a decal provided on a portion of the substrate as a marker forlabeling the substrate, the decal having a different color than that ofthe substrate and having good edge definition after a heat cycle duringwhich the ceramic member is exposed to a temperature of at least 1100°C. wherein the decal consists essentially of a refractory ceramiccomposition including unstabilized zirconia and silica.
 11. The ceramicmember of claim 10, wherein the unstabilized zirconia and the silica arepresent at an unstabilized zirconia:silica weight ratio of from 9:1 to1:1.
 12. The ceramic member of claim 11, wherein the unstabilizedzirconia and the silica are present at an unstabilized zirconia:silicaweight ratio of from 4:1 to 2:1.
 13. A method for labeling a ceramicmember, comprising: applying a decal on a portion of a ceramic substrateas a marker for labeling the substrate, the substrate comprising siliconcarbide or silicon nitride; and thereafter heat treating the ceramicsubstrate after applying the decal to a temperature of at least 1100°C., the decal providing good edge definition after heat treating andhaving a different color than the substrate.
 14. The method of claim 13,wherein the decal maintains good edge definition after repeated heatingcycles to a temperature of at least 1100° C.
 15. The method of claim 13,wherein the decal is applied to the ceramic substrate in an unfiredstate.
 16. The method of claim 13, wherein the decal has a thickness notless than about 20 microns.
 17. The method of claim 16, wherein thedecal has a thickness not less than about 50 microns.
 18. The method ofclaim 13, wherein the decal has good contrast with the ceramicsubstrate.
 19. The method of claim 13, wherein the decal remains stable,having good adhesion to the ceramic substrate after the heat treating.20. The method of claim 13, wherein the decal has clean lines that donot bleed into the ceramic substrate and maintains good contrast withthe ceramic substrate.
 21. The method of claim 13, wherein the decal iscomprised of a fired colored ink.
 22. The method of claim 13, whereinthe ceramic substrate comprises silicon carbide or silicon nitride. 23.The method of claim 22, wherein the substrate comprises silicon carbide.24. A method for labeling a ceramic member, comprising: applying a decalon a portion of a ceramic substrate as a marker for labeling thesubstrate; and thereafter heat treating the ceramic substrate afterapplying the decal to a temperature of at least 1100° C. the decalproviding good edge definition after heat treating and having adifferent color than the substrate, wherein the decal consistsessentially of a refractory ceramic composition including unstabilizedzirconia and silica.
 25. The method of claim 24, wherein theunstabilized zirconia and the silica are present at an unstabilizedzirconia:silica weight ratio of from 9:1 to 1:1.
 26. The method of claim25, wherein the unstabilized zirconia and the silica are present at anunstabilized zirconia:silica weight ratio of from 4:1 to 2:1.